The present invention mainly relates to a so-called weak current wire such as a non-underground communication cable, a subscriber lead-in telephone cable, i.e., an overhead communication cable, a wire within a building or paired telephone wires used in a personal house, a signal cable, a control cable, a wire harness for aircraft, or a wire harness for motor car.
In order to make telephone conversation clear, communication cables (telephone cables) require to have such characteristics that (1) transmission loss lessens, (2) crosstalk lessens, (3) transmission frequency has a certain range and (4) they are hard to be susceptive to external induction.
In order to achieve this object, no problem arises as to external induction in the case of an underground distribution method because complete shielding is provided. However, since its laying cost is as expensive as 5 to 10 times compared with an overhead distribution method, the overhead distribution method has been used in no small numbers. However, the overhead distribution method is susceptive to influence of induction, temperature change or the like and offers a problem by the proximity to or crossing with a power line. In addition, such complete shielding as in an underground cable is not made in a house wire, personal paired telephone wires for subscriber or the like, and so it is affected by electromagnetic waves generated from a nearby power line and various electric appliances such as refrigerators, facsimiles, televisions, personal computers and air-conditioning equipment, and talking trouble may be incurred in some cases.
In Japan, alternating electrification of respective JRs, increase in transmission capacity of general transmission lines or the like is conducted. Inductive interference against communication cables from such external induction sources becomes a problem. Induction includes electrostatic induction and electromagnetic induction. The electrostatic induction is induction from voltage from a power line or the like, and the electromagnetic induction is induction by a magnetic flux made by an electric current. In order to shield the electrostatic induction, it is only necessary to earth a metal sheath that a cable has, and it has heretofore scarcely become a problem. However, the influence thereof has come to be nonnegligible in communication devices having no shield, such as lead-in wires, with the build-up of transmission voltage in recent years.
The electromagnetic induction is then considered. It is only necessary to lengthen isolation from a power line and shorten a distance being in parallel. Even on the communication cable side, a member low in both earthing resistance and electric resistance is used as a shielding member (for earthing) to make an electric current (electric current acting in a direction negating induction voltage to a cable conductor from the power line) flowing in the cable shielding member high, thereby negating external induction to the cable conductor. In order to make induction voltage to the cable conductor high by the electric current flowing in the cable shielding member, it is necessary to make the magnetic permeability of the shielding member high so as to enlarge a magnetic flux by the electric current.
Cables equipped with the former cable shielding member, through which a high electric current flows, include cables longitudinally provided with an aluminum tape along a cable conductor and aluminum-coated cables, and a steel band sheath is considered as the latter shielding member capable of making the magnetic permeability high. In order to completely shield a cable conductor by these shield members, however, the cable conductor must be coated cylindrically with the prescribed metal, and so the outer diameter and weight of the resulting cable are increased, and the flexibility thereof becomes poor. Therefore, such a cable may be not preferable in some cases. In particular, taking the influence of recent electromagnetic wave environment on communication cables into consideration, there is an eager demand for appearance of a cable shield (including shield for lead-in paired wires) which is simple and rich in flexibility and does not cause communication trouble.
Further, signal cables, control cables, wire harnesses for aircraft or motor car are only slightly affected by external electromagnetic waves, and no problem arise thereon in an ordinary case. However, it is considered that electromagnetic waves radiated by themselves may affect various measuring instruments or electronic apparatus such as monitors. Therefore, a shield for preventing radiation of electromagnetic waves is required thereof. However, such a cable has heretofore been often used without providing any particular shield. When a shield is required, it is considered that a metal tube or braided shield is used like general communication cables. However, even in this case, the same problems as described above arise on shielding. Accordingly, it goes without saying that the cable shield which is simple and rich in flexibility and does not cause communication trouble is preferred.
As a shield layer for achieving the above object, no established shield flexible, lightweight and having a small outer diameter has heretofore existed. For example, in the case where a cable conductor is a coaxial line, a vinyl coating is provided as needed, and an Aldrey or iron wire-braided armor is provided.
This braid is very troublesome in production, a special braider must be provided, and production rate is also slow. Therefore, it involves a problem that its cost becomes high. It is also considered that a laminate tape of a metal and a plastic is wound. However, flexibility becomes poor in the case of longitudinal covering, and the thickness thereof is greater compared with paper, and so increase in outer diameter has been unavoidable.
The present invention solves the above-described problems.
Incidentally, the non-underground communication cables are defined as including subscriber lead-in telephone cables in the present specification. The cable conductors are also defined as including paired telephone wires.
A weak current wire according to a first aspect of the present invention is a communication cable or weak current wire selected from non-underground communication cables, signal cables, control cables and wire harnesses including cables for communication, signal and control, which is characterized in that around a cable conductor or a wire harness, an electromagnetic wave shield layer of a porous sheet made by making a metallic fiber sheet from a slurry containing metallic fibers by a paper machine and sintering the metallic fiber sheet is provided.
In the weak current wires according to the first aspect of the present invention, the electromagnetic wave shield layer may be provided through an adhesive layer around the cable conductor or the wire harness. Further, dot-like fine through-holes may be provided in the adhesive layer.
A weak current wire according to a second aspect of the present invention is a weak current wire selected from non-underground communication cables, signal cables, control cables and wire harnesses including cables for communication, signal and control, which is characterized in that around a cable conductor or a wire harness, an electromagnetic wave shield layer of a porous sheet made by making an unsintered metallic fiber sheet from a slurry containing metallic fibers by a paper machine and impregnating or filling the unsintered metallic fiber sheet with a thermosetting conductive adhesive, or of a laminated porous sheet made by laminating a thermosetting conductive adhesive layer on at least one side of the unsintered metallic fiber sheet is provided.
A weak current wire according to a third aspect of the present invention is a weak current wire selected from non-underground communication cables, signal cables, control cables and wire harnesses including cables for communication, signal and control, which is characterized in that around a cable conductor or a wire harness, an electromagnetic wave shield layer of a porous sheet made by making a metallic fiber sheet from a slurry containing metallic fibers by a paper machine, sintering the metallic fiber sheet and impregnating or filling the sintered metallic fiber sheet with a thermosetting conductive adhesive, or of a laminated porous sheet made by laminating a thermosetting conductive adhesive layer on at least one side of the sintered metallic fiber sheet is provided.
In the weak current wires according to the second and third aspects of the present invention, a rubber component may be contained in the thermosetting conductive adhesive. Further, a rubber component and an antioxidant may be contained in the thermosetting conductive adhesive.
In the weak current wires according to the first to third aspects of the present invention, specific examples of the weak current wires include (1) a weak current wire wherein the metallic fiber sheet is made of metallic fibers having a fiber length of 1 to 10 mm and a fiber diameter of 1 to 20 xcexcm, and has a basis weight of 30 to 500 g/m2, (2) a weak current wire wherein the metallic fiber sheet has voids of 50 to 93%, (3) a weak current wire wherein the non-underground communication cable is an overhead communication cable, (4) a weak current wire wherein the non-underground communication cable is a coaxial line, (5) a weak current wire wherein the wire harness is either a wire harness for aircraft or a wire harness for motor car, and (6) a weak current wire wherein the electromagnetic wave shield layer of the porous sheet is made by dispersing conductive metallic fine powder or magnetic metallic powder in metallic fibers and subjecting the resultant dispersion to sheet making.
According to the present invention, there can be provided cables not susceptive to external electromagnetic waves by using a shield, which is simple without using any complete shielding member such as, for example, a copper pipe, thin like paper and obtained without adopting any inefficient production method such as braiding, in communication cables and other weak current wires. In some cases, some signal cables, control cables and wire harnesses may affect measuring instruments or electronic apparatus such as image monitors by radiating electromagnetic waves to the outside. According to the present invention, however, a shielding effect can be achieved with relative ease by providing the shield used in the present invention on the outsides of these signal cables, control cables and wire harnesses while lessening increase in weight and volume.